1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method, and more particularly, to those characterized in its process for making a gate oxide.
2. Related Background Art
NO or N2O gas is typically used as the source material gas for making a nitrogen oxide film as a process for manufacturing a semiconductor. In this case, a maximum peak of nitrogen concentration distribution is formed near a Si substrate.
However, since nitrogen has a fixed charge, there occurs the problem that, if the nitrogen concentration is too high, in the Si side, it causes transistors fluctuate in threshold value, and causes a decrease of the channel current.
Taking it into consideration, for the purpose of locating the peak of the nitrogen concentration away from a boundary face of the Si substrate, re-oxidation is required normally after the nitrogen oxide film is formed. In this case, however, a decrease in nitrogen quantity in the film due to the re-oxidation or non-uniform re-oxidation may cause deterioration of the gate reliability.
That is, it was difficult to arbitrarily control the nitrogen concentration and its distribution with conventional manufacturing methods of semiconductor devices.
On the other hand, upon making a gate oxide film of a flash memory, a nitrogen oxide film made by using NH3 gas as the source material gas is often used.
In case of using NH3 gas as the source material gas, the nitrogen profile in the resulting oxide film is characterized in having maximum peaks of the nitrogen concentration distribution both on the surface of the nitrogen oxide film and near the Si substrate. This property is effective for reducing trapping of electrons and holes in a device like a flush memory configured to injecting electrons from two directions, i.e. from its gate electrode and from the Si substrate.
However, the use of NH3 as the source material gas causes the problem that hydrogen is introduced in addition to nitrogen into the nitrogen oxide film.
Since hydrogen behaves as an electron trapping site, it must be minimized to increase the reliability of the gate oxide film. For this purpose, it is necessary to insert re-oxidation after deposition of the nitrogen oxide film. In this case, however, an impurity doped into the Si substrate in a preceding step may spread and fluctuate the threshold voltage of the transistors, and may decrease the channel current. Furthermore, nitrogen may spread externally during the additional oxidation, which may decrease the nitrogen concentration near the surface of the nitrogen oxide film and may disable to obtain required electric properties.
That is, even when using NH3 gas, it was still difficult to arbitrarily control the nitrogen concentration and its distribution with conventional manufacturing methods of semiconductor devices.
In conclusion, conventional manufacturing methods of semiconductor devices involved difficulties in arbitrarily controlling nitrogen concentration and its distribution because of employing the process of first fabricating a nitrogen oxide film by using NO, N2O or NH3 gas and thereafter re-oxidizing it to make a gate oxide film.
It is therefore an object of the invention to provide a semiconductor device and its manufacturing method capable of controlling nitrogen concentration and its distribution by increasing the nitrogen concentration near its boundary face with an electrode in a process of making a nitrogen oxide film using a gate oxide film.
According to an aspect of the invention, there is provided a semiconductor device comprising:
a semiconductor substrate;
a gate oxide film formed on the semiconductor substrate; and
first transistors each having a first gate formed on the gate oxide film and a pair of source/drain formed in confrontation in the semiconductor substrate,
the gate oxide film having a higher nitrogen concentration in portions thereof nearer to the first gates than that of a portion thereof nearer to the semiconductor substrate.
In another aspect of the invention, the gate oxide film has a higher nitrogen concentration in a portion thereof near the first gate than in a portion thereof near the semiconductor substrate.
According to a further aspect of the invention, there is provided a method for manufacturing a semiconductor device including first transistors each having a first gate on a gate insulating film on a semiconductor substrate, in which the gate insulating film is fabricated by a process comprising:
a first step of making a first oxide film on the semiconductor substrate;
a second step of making a silicon film on the first oxide film;
a third step of making a first nitrogen oxide film on the silicon film;
a fourth step of segregating nitrogen along a surface of the first nitrogen oxide film; and
a fifth step of oxidizing the silicon film to form a second nitrogen oxide film from the first oxide film, the silicon film and the first nitrogen oxide film.
According to a still further aspect of the invention, there is provided a method for manufacturing a semiconductor device including first transistors each having a first gate on a gate insulating film on a semiconductor substrate, in which the gate insulating film is fabricated by a process comprising:
a first step of making a first nitrogen oxide film on the semiconductor substrate;
a second step of making a silicon film on the first nitrogen oxide film;
a third step of making a second nitrogen oxide film on the silicon film;
a fourth step of segregating nitrogen along a bottom surface of the first nitrogen oxide film and along a top surface of the second nitrogen oxide film; and
a fifth step of oxidizing the silicon film to form a third nitrogen oxide film from the first nitrogen oxide film, the silicon film and the second nitrogen oxide film.